json: fix ptr field access (#17690)

vlib/json/json_encode_with_mut_test.v

@@ -0,0 +1,42 @@
+module main
+
+import json
+
+pub enum PlatformType {
+	unknown
+	osx
+	ubuntu
+	alpine
+}
+
+pub enum CPUType {
+	unknown
+	intel
+	arm
+	intel32
+	arm32
+}
+
+[heap]
+pub struct Node {
+pub:
+	name string = 'mymachine'
+pub mut:
+	platform    PlatformType
+	cputype     CPUType
+	done        map[string]string
+	environment map[string]string
+}
+
+pub fn (mut node Node) save() ! {
+	data := json.encode(node)
+	dump(data)
+}
+
+fn test_encode_with_mut_struct() {
+	mut n := Node{
+		platform: .osx
+		cputype: .unknown
+	}
+	n.save() or { panic(err) }
+}